Parasitoids that have been mass-reared and released to control pests in field crops such as alfalfa, corn, cotton, sorghum, soybean, sugarcane, and rice are from 2 orders, Hymenoptera and Diptera, with the majority of species from the genus Trichogramma. Release rates for inoculative release are considerably lower than those for inundative release but require an adequate host population in the field for parasitoid propagation. Methods to provide a precocious host population in the field include planting nurse crops to attract the host, using early-season alternate hosts, trapping hosts for parasitoid propagation, manipulating growth of the crop to support a precocious host population, and concurrent release. Reasons for the wide range of release rates that have been used are discussed, and the advantages and disadvantages of different release methods are presented. The distribution of parasitoids during a release is a major part of the cost, and reduction in number of release points used appears justified. Direct observation, light traps, pheromone traps, and degree-day accumulation have been used to determine the timing of releases, but pheromone trapping appears to be the most suitable method, especially in situations where there is considerable variation in pest abundance among fields.